Electrowetting display devices are known. In an off state of such a display device an oil covers the area of a pixel of the display. In an on state the oil is retracted so as not to cover the pixel area. To switch the pixel into the on state a voltage is applied across the oil via an electrically conductive fluid immiscible with the oil, which exerts an electrostatic force on the conductive fluid, and to switch the pixel into the off state the voltage is switched off. Thus, for example, where the oil is coloured, a radiation beam passing through the pixel when in the off state passes through the oil and is imparted with colour, but does not pass through the oil with the pixel in the on state, and so its colour remains substantially unchanged.
In such an electrowetting device, the phenomenon of backflow is known. Backflow is where, despite the voltage being switched on and continuously applied, the oil tends to flow back from the on state to eventually adopt the off state. Due to backflow, disadvantageously, a desired optical effect which would be provided in a radiation beam by the oil in the on state is not maintained. Instead, an undesired optical effect corresponding to the oil being in the off state, or in an intermediate state between the on and off states is provided.
Techniques are known for reducing backflow. For example, using a compound such as a dye with approximately no overall polarity has been found to reduce backflow. However, for asymmetric dye compounds for example, it can be more difficult to provide a compound with approximately no overall polarity.
It is desirable to provide further improvements for electrowetting apparatus, for example for reducing backflow.